It is known, for example, from the disclosure in French Pat. No. 1,555,564, that the reaction of difunctional amines with at least one equimolar amount of a difunctional imide of an unsaturated dicarboxylic acid provides so-called polymide prepolymers. The prepolymers, on further heating, produce a family of macromolecular polyimides with outstanding resistance to heat and solvents and with high mechanical strength and excellent electrical properties.
Such prepolymers are disclosed to be formed by reacting a difunctional amine of the general formula: EQU H.sub.2 N -- R -- NH.sub.2
wherein R is a divalent group having up to about 30 carbon atoms with a difunctional imide of the general formula ##STR1## wherein R.sub.1 is a divalent group containing up to about 30 carbon atoms and having at least one carbon-to-carbon double bond. R and R.sup.1 can be the same or different.
It is believed that the predominant reaction in the formation of such pre-polymers is the addition of the elements of an amine group across the double bond in R.sup.1. However, a second type of reaction is also known to occur, that is, vinyl addition polymerization between the unsaturated carbon-to-carbon linkages in the groups designated R.sup.1 in the above formula.
It is further known that the reaction between the difunctional amine and the difunctional imide may be carried out either in bulk or in an inert polar carrier medium. In the bulk process, apparently because reactivity is low unless the mixture is melted, it is necessary to heat the reaction mixture to an elevated temperature, of the order of 100.degree. to 200.degree. C., to induce any reaction at all between the two reagents. In most cases at least about 120.degree. C. is used and it is reported to be preferred to heat at about 160.degree. C. if the reaction is to be completed in a reasonable time. If an inert polar carrier medium is to be used it is suggested to employ dimethylformamide (b.p. 153.degree. C.), N-methylpyrrolidone (b.p. 202.degree. C.) and dimethylactamide (b.p. 163.degree.-165.degree. c). If dimethylformamide is used as a carrier it is specifically recommended to heat the solution at 150.degree. C., i.e., near the reflux temperature. Thus the reaction, even in a polar carrier medium, is reported to proceed at a temperature from the middle to the upper range of temperature necessary for the bulk process.
Although the advantages are self-evident, there have not been proposed any successful processes to prepare such prepolymers by directly reacting a diamine with an anhydride of an unsaturated dicarboxylic acid. It has always been the practice to prepare a difunctional imide first and then to isolate it, further purify it, and finally to react the dry, pure imide with more amine.
It is known that production of the imide by reaction of a diamine with an anhydride is accompanied by the formation of various, and often substantial, quantities of side reaction products. Conventionally these are removed by precipitation of the imide and other separation techniques and the imide is isolated and dried before being reacted with more diamine. It has been thought to be essential to purify the imide in order to obtain uniformity in the prepolymers.
It has now been found, however, that with careful attention to reactant ratios, the use of reaction temperatures below those suggested in the prior art, and if the process is carried out in a solvent, the polyimide prepolymers can be obtained by directly reacting a diamine and an anhydride.
Even though during the process the imide and side reaction products are produced, it is not necessary to separate the imide from the side products. Instead, all of the products are surprisingly further reactable with the amine, in situ, to form a novel polyimide resin.
As will be seen, with appropriate starting materials, overall yields of greater than 95% can be obtained. In contrast, in processes where the imide is isolated, yields are usually only about 80%, both based on raw materials, because about 15% or more of the raw materials are lost to side products.
The novel polyimide resins, obtained from mixtures containing the side products, have been found to be different from those that are made from purified, isolated and dried imide. Contrary to what would be expected, they tend to be substantially more thermally stable than those prepared by the prior art bulk or solvent techniques outlined above.
It is, therefore, a primary object of this invention to provide polyimide prepolymers directly by reacting a difunctional amine and an anhydride of an unsaturated dicarboxylic acid.
The present invention also has as a primary object the provision of a process to provide novel and improved polyimides.
Still another object of this invention is to provide polyimide prepolymers by a process that is economical and safe.
A further object of this invention is to provide polyimide prepolymers which can be thermally converted to macromolecular polyimides with better flow, molding and physical properties than those made by prior art bulk processes.